Greece recently defaulted in their debts and upon the news, many sources are touting that they are the first 'developed nation' to do so. Why are they a part of such an elite club and what did they do to earn such an honor?
The news reports are probably using 'Developed Country' as a substitute for the IMF classification of 'Advanced Economy' that includes 37 countries and the list of which you can find here.
You can find the reasoning the IMF uses as part of their FAQ here.
To quote the FAQ in part:
The main criteria used by the WEO to classify the world into advanced economies and emerging market and developing economies are (1) per capita income level, (2) export diversification—so oil exporters that have high per capita GDP would not make the advanced classification because around 70% of its exports are oil, and (3) degree of integration into the global financial system.
According to the Excel file provided by the IMF here the group compositions have not changed since at least 1990, and the IMF does not provide analysis from before 1980, since the data are so unreliable, so Greece has been considered an 'Advanced Economy' since at least 1990, and likely since the IMF came up with the definition.
This is really more of an economics question, though, so I encourage you to cross-post this to /r/AskSocialScience or even /r/askscience.